<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let j = prompt("请输入一个数")
        let k = 0
        for(let i=1;i<=j;i++){
            k+=i
        }
        document.writeln(k)
    </script>
</body>
</html>